This invention relates to the field of sensors and particularly to integrated circuit, monolithic accelerometers.
Photolithographic processes can be used to fabricate both sensors and interface circuits on a single silicon substrate. These monolithic devices can be made incredibly small and can provide much faster responses than the larger discrete sensor with separate electronic processing. Such monolithic sensors have been developed to measure light intensity, temperature, pressure, gas dynamics, and acceleration.
Monolithic sensors are particularly suitable for use in measuring acceleration. Such accelerometers can be made very small to reduce the load created in the accelerating vehicle and to provide fast response to any changes in acceleration. In addition, small monolithic accelerometers can be constructed to withstand the large forces (over 30,000 g) which are developed at extremely high accelerations.
Cantilever beams may be used as the stress generating member of monolithic accelerometers. Thin cantilever beams of silicon dioxide and p.sup.+ silicon have been isolated from a silicon substrate by removing the substrate from below a portion of a layered structure consisting of silicon dioxide and heavily doped p-type silicon (R. D. Jolly and R. S. Muller, J. of Elec. Chemical Soc., Vol. 127, No. 12, December, 1980). The etchant used to remove the substrate from the overlying beam structure was ethylenediamine, pyrocatechol, and water. This etchant, and other anisotropic etchants have also been proposed for processing silicon devices by Kenneth E. Beam in a paper entitled "Anisotropic Etching of Silicon" in the IEEE Transactions on Electron Devices, Vol. Ed-25, No. 10, October, 1978.
For very high accelerations, sturdier beams etched from the silicon substrate itself are needed. These sturdier beams can utilize the complete thickness of the silicon wafer rather than just a layered structure above the substrate such as described above. However, the etchant used and the process followed must be compatible with the processes required to construct the integrated circuit on the same chip as the beam. This has required the use of complicated etching processes which require many closely controlled protection and etching steps.
One method which has been used to control the depth of etching uses an epitaxially grown etch-stop layer of n-silicon during electroetching (U.S. Pat. No. 4,498,342). Another method uses a p-n junction formed in silicon to define the etching pattern of a diamine, water, and catechol etchant (U.S. Pat. No. 4,305,298). In some methods a silicon oxide layer is apparently used to stop etching with a sodium hydroxide etchant (U.S. Pat. No. 4,071,838).